I’ve been using Insteon for years but was thinking of giving z-wave a try. I’ve got a storage building that I thought I might install a z-wave door sensor in for a simple security system. The z stick gen 5 shows an open air range of 100 meters (328 feet) but that doesn’t really tell me anything. Is it reasonable to expect the stick to transmit/receive through 2 plaster and vinyl walls and a distance of 50-60 feet with no other z-wave devices between?

Some devices are better than others, but it is highly doubtful that a single device will work that far. You will either need to use a repeater or another Z-wave device in between. I'd go with another Z-wave device; the repeaters are the same price as most standard devices are.

The biggest issue is devices that need to be powered are vulnerable to water getting in them. The Water Sensor 6 is only IP65 when used without the optional dock; add the dock in and it is no longer IP65 rated.